Amphiphilic block copolymers comprising a hydrophobic block and a hydrophilic block have been well studied in recent years, because of their capacity for self-assembly into a variety of nanostructures as the surrounding solvent is varied. See Cameron et al., Can. J. Chem./Rev. Can. Chim. 77:1311-1326 (1999). In aqueous solutions, the hydrophobic compartment of an amphiphilic polymer has a tendency to self-assemble in order to avoid contact with water and to minimize the free interfacial energy of the system. At the same time, the hydrophilic blocks form a hydrated “corona” in the aqueous environment, and so the aggregates maintain a thermodynamically stable structure. The result is a stable, latex-like colloidal suspension of polymer aggregate particles having hydrophobic cores and hydrophilic coronas.
Comb-type amphiphilic co-polymers differ from block co-polymers in that the backbone is largely hydrophobic or hydrophilic, with polymer chains of opposite polarity pendant from the backbone rather than incorporated into it. Comb-type copolymers have been prepared with hydrophobic backbones and hydrophilic branches (Mayes et al., U.S. Pat. No. 6,399,700), and also with hydrophilic backbones and hydrophobic branches (Watterson et al., U.S. Pat. No. 6,521,736; Uchegbu et al., U.S. Application Publication No. 2006/0148982). The former were used to provide multivalent presentation of ligands for cell surface receptors, while the latter were used to solubilize drugs and deliver them to cells.
Amphiphilic polymer aggregates have been studied as carriers for solubilizing insoluble drugs, targeted drug delivery vehicles, and siRNA or gene delivery systems. They have a more stable structure than conventional low-molecular-weight micelles, due to chain entanglement and/or the crystallinity of the interior hydrophobic region. The polymeric nature of the vehicle renders the aggregates relatively immune to the disintegration that ordinary liposomes suffer when diluted below their critical micelle concentration. The absence of a bilayer membrane enables them to more readily fuse with cell membranes and deliver their payload directly to the cell. The amphiphilic nature of the aggregates also confers detergent-like activity, and appropriately targeted aggregates appear to be capable fusing with and disrupting viral coat proteins.
Due to the excellent biocompatibility of poly(ethylene glycol) (PEG), and the apparent ability of PEG-coated “stealth” particles to evade the reticuloendothelial system, micelles, liposomes, and polymers incorporating PEG have been extensively considered as materials for drug delivery systems. There are many reports of the use of PEG as the hydrophilic component of PEG-lipids (forming liposomes and micelles); see for example Krishnadas et al., Pharm. Res. 20:297-302 (2003). Self-assembling amphiphilic block copolymers, which self-assemble into the more robust “polymersomes”, have also been investigated as vehicles for drug solubilization and delivery. See for example Jones and Leroux, Eur. J. Pharm. Biopharm. 48:101-111 (1999); Photos et al., J. Controlled Release, 90:323-334 (2003); Kataoka et al., Adv. Drug Deliv. Rev. 47:113-131 (2001); and Torchilin, J. Controlled Rel. 73:137-172 (2001).
See also Gref et al., Int. Symp. Controlled Release Mater. 20:131 (1993); Kwon et al., Langmuir, 9:945 (1993); Kabanov et al., J. Controlled Release, 22:141 (1992); Allen et al., J. Controlled Release, 63:275 (2000); Inoue et al., J. Controlled Release, 51:221 (1998); Yu and Eisenberg, Macromolecules, 29:6359 (1996); Discher et al., Science, 284:113 (1999); Kim et al., U.S. Pat. No. 6,322,805; Seo et al., U.S. Pat. Nos. 6,616,941 and 7,217,770; Seo et al., European Patent No. EP 0583955. The use of poly(ethyleneimine) (PEI) in this capacity has also been reported, with a focus on delivery of oligonucleotides (Nam et al., U.S. Pat. No. 6,569,528; Wagner et al., U.S. Patent application publication No. 20040248842). In a similar vein, Luo et al., in Macromolecules 35:3456 (2002), describe PEG-conjugated polyamidoamine (“PAMAM”) dendrimers suitable for delivery of polynucleotides.
In addition to the need to solubilize, distribute, and deliver drugs, there is a need for targeted drug delivery systems that home in specifically on a target cell type, tissue, tumor, or organ. This is usually accomplished by attachment of antibodies or other ligands with a specific affinity for cell walls at the target site. However, PEG lacks functional groups except at the ends of the polymer chains, and in a block copolymer the majority of the terminal groups are inevitably taken up by bonds to the other block copolymer component. For this reason, attachment of targeting moieties such as antibodies or cell-adhesion molecules to PEG block copolymers is generally limited to the non-PEG block, which unfortunately is not the part of the copolymer that is normally exposed in the corona of the self-assembled aggregate.
The phase separation phenomenon which results in the self-assembly of block copolymers into polymer aggregates is readily reversible, and attempts have been made to increase the stability of the aggregates by cross-linking the hydrophobic core (see European Patent No. EP 0552802). Covalent attachment of the drug to the hydrophobic component of a block copolymer has also been attempted (Park and Yoo, U.S. Pat. No. 6,623,729; European Patent No. EP 0397307).
Dendritic polymers are readily conjugated to targeting moieties, and also have the potential to target specific cells in vivo (Singh et al. (1994) Clin. Chem. 40:1845) and block adhesion of viral and bacterial pathogens to biological substrates. Comb-branched and dendrigraft polymers conjugated to multiple sialic acid have been evaluated for their ability to inhibit virus hemagglutination and to block infection of mammalian cells in vitro (Reuter et al. (1999) Bioconjugate Chem. 10:271). The most effective virus inhibitors were the comb-branched and dendrigraft macromolecules, which showed up to 50,000-fold increased activity against these viruses.
Recently, the pharmaceutical company Starpharma announced the successful development of a dendrimer-based biocide (VivaGel™) that prevents HIV infection by binding to receptors on the virus's surface (Halford (2005) Chem. & Eng. News 83 (24):30). Chen at al. (2000) (Biomacromolecules. 1:473) have reported that quaternary ammonium functionalized poly(propyleneimine) dendrimers are very potent biocides.
Killing cancer cells without damaging the patient's nearly-identical healthy cells is a particularly difficult proposition. Even with the most successful chemotherapeutic agents, mechanism-based selective toxicity toward cancer cells is only partially attained. For this reason, cancer therapeutics are a class of agents for which targeted delivery is especially desirable, and a great deal of effort has gone into developing ligands for cancer-specific cell surface markers. (Delgado and Francis, Drug Targeting: Strategies, Principles and Applications, Humana Press, 2000)
For example, the cell surface receptor for folic acid is often elevated in cancers of the ovary, kidney, lung, breast, brain, and endometrium, and in myeloid cells of hematopoietic origin. Because the folate receptor is usually cryptic in normal cells, but is displayed on the surface of cancer cells, it has frequently been exploited as a target for receptor-directed cancer therapies (Lu and Low, J Control Release. 91:17-29 (2003)). Conjugates of folic acid directly with antineoplastic drugs, antibodies (U.S. Pat. No. 5,547,668), liposomes (Liu and Lee, Drug Design Reviews Online, 2:547-552 (2005)), and other nanoparticulate drug delivery constructs (Torchilin, Adv. Drug Delivery Rev. 58:1532-1555 (2006)) are among the reported applications. Micelles formed from folate-conjugated amphiphilic block copolymers have been shown to selectively deliver paclitaxel to tumor cells (Park et al., J. Controlled Release 109:158-168 (2005)).
Similarly, the epidermal growth factor receptor (ErbB1, EGFR) is overexpressed in a wide spectrum of human tumors of epithelial origin, including breast, head and neck, gastric, colorectal, esophageal, prostate, bladder, renal, pancreatic, and ovarian cancers, and non-small cell lung cancer. These findings have established EGFR as another important target for receptor-mediated delivery systems. EGF itself exhibits strong mitogenic and angiogenic activity, which makes it unsuitable as a targeting moiety, but a variety of non-agonist ligands for EGFR have been developed for this purpose.
Antibodies directed against cell-specific or tumor-specific epitopes have been used successfully as targeted therapies, either alone (to activate the patient's complement system) or to deliver radioisotopes or toxins. For example, tositumomab, a murine IgG2a lambda monoclonal antibody directed against the CD20 B-lymphocyte antigen, may be radioiodinated and used to deliver iodine-131 selectively to lymphoma cells. This has proved successful in the clinic as a treatment for non-Hodgkin's lymphoma, and has been commercialized for this indication under the trade name Bexxar™. Similarly, ibritumomab (Zevalin™), another anti-CD20 monoclonal antibody, has been used to deliver yttrium-90 for immunoradiotherapy of non-Hodgkin's lymphoma.
Other clinically-successful cancer-targeting antibodies include alemtuzumab (anti-CD52, Campath™) for chronic lymphocytic leukemia; bevacizumab (anti-VEGF, Avastin™) for colon cancer and lung cancer; cetuximab (anti-EGFR, Erbitux™) and panitumumab (anti-EGFR, Vectibix™) for colon, head and neck cancer; gemtuzumab (anti-CD33, Mylotarg™) for acute myelogenous leukemia; rituximab (anti-CD20, Rituxan™) and epratuzumab (anti-CD22, Lymphocide™) for non-Hodgkin's lymphoma, and trastuzumab (anti-HER-2, Herceptin™) for breast cancer. Of special relevance is the immunotoxin Mylotarg™, in which an anti-CD33 antibody is conjugated to calicheamicin, a cytotoxic antitumor drug.
There remains a need for a drug delivery system that is stable, biocompatible, amenable to the attachment of a variety of targeting moieties, and efficient at delivering a substantial payload of drug to the desired tumor cell targets. There is also a need for targeted anticancer agents that are similarly stable, efficient, and biocompatible.